Faecal collection methods result in different estimates of nutrient apparent digestibility in Penaeus monodon

Accurate measurement of apparent digestibility (AD) is essential to characterise the nutritive value of ingredients in aquaculture. For crustaceans, current methods rely on collection of excreted faeces by settlement. Excreted faeces lose soluble nutrients through leaching, leading to an over-estimation of AD. An alternative approach is to remove faeces by dissecting them from the hindgut prior to evacuation. In this study, we compared estimates of nutrient AD of ingredients in Penaeus monodon from three faecal collection methods: faeces obtained by dissection, faeces collection onto a screen (FS) and faeces collection into a bucket which is then centrifuged in freshwater and decanted (FB). Furthermore, we dissected faeces from three gastro-intestinal tract regions: foregut, proximal hindgut and distal hindgut, to test the validity of the hypothesis that digestion and absorption do not occur beyond the hepatopancreas of P. monodon. Protein and amino acid AD of protein-rich ingredients were significantly different between methods. On average, estimates of protein AD of ingredients obtained using dissection were 15.2% and 27.0% lower compared to the FS and FB methods respectively (AD values of 60.6% vs. 71.5% and 83.0% respectively). Similar trends were observed regarding amino acid (AA) AD of ingredients, where values of AD by dissection were consistently lower than the other two methods. The range of protein and AA AD values observed for different ingredients was greater when using dissection than the other methods. Potentially, dissection allows for greater sensitivity in detecting differences in digestibility of protein ingredients. In contrast, AD estimates of dry matter, starch and lipid were not affected by sampling method. These results suggest that AD of protein and AA is significantly over-estimated using traditional faecal collection methods and that the standard practices for evaluating digestible protein of protein-rich ingredients in shrimp needs to be reconsidered.